obazl v2 migration: opam pkg labels

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Mapping from opam pkg labels to bazel labels:

• single segment: @pkg//:pkg. E.g. ounit2 => @ounit2//:ounit2

• multiple segments: seg1.seg2.seg3 => @seg1//seg2/seg3. E.g. ptime.clock.os => @ptime//clock/os

Special cases: the OPAM repo contains a handful of "pseudo-packages", which are effectively aliases for files included in the standard OCaml distribution. In OPAM switches these packages contain nothing but a META file that defines a package (and possibly subpackages) whose content properties point to the OCaml standard lib (which is $OPAM_SWITCH_PREFIX/lib/ocaml). I think they're retained for backward compatibility.

In OBazl we do not need such pseudo-packages; we just (pre)define targets in the @ocaml namespace. For example, instead of dynlink => @dynlink//:dynlink we have @ocaml//dynlink. This means that such "packages" cannot be overridden using e.g. --override_repository, but that should not be a problem; since they are included in the core distribution, devs should have no reason to override them.

So a conversion tool needs to special-case these mappings:

• bigarray => @ocaml//bigarray
• compiler-libs => @ocaml//compiler-libs
• compiler-libs.common => @ocaml//compiler-libs/common
• compiler-libs.bytecomp => @ocaml//compiler-libs/bytecomp
• compiler-libs.optcomp => @ocaml//compiler-libs/optcomp
• compiler-libs.toplevel => @ocaml//compiler-libs/toplevel
• compiler-libs.native-toplevel => @ocaml//compiler-libs/native-toplevel
• dynlink => @ocaml//dynlink
• str => @ocaml//str
• unix => @ocaml//unix

Threads are slightly more complicated. Historically we had threads.posix and threads.vm; now there is just `threads.

• threads.posix => @ocaml//threads
• threads.vm => @ocaml//threads (the vmthreads code no longer exists)

Note that these targets within the @ocaml namespace do not necessarily correspond to subdirectories in the opam switch. In fact most of them are aliased to separate repos. For example, the compiler-lib targets alias to @ocaml.compiler-libs. The intention is that the @ocaml labels provide a simple, uniform, idiomatic user interface, that does not use '.' in repo names. The repos they alias to include links, which makes globbing more straightforward. This is needed because each ocaml_import target needs to glob all the deps of the main (archive) files it provides.

[tek]

awesome